1. Field of the Invention
The exemplary embodiments of the present invention relate to a substrate transport system and, more particularly, to a substrate transport system capable of orienting and aligning a substrate “on the fly”.
2. Brief Description of Earlier Related Developments
In semiconductor fabrication processes, wafers are transferred between stations, such as storage, queuing, processing and other work stations. In typical automated wafer handling processes, a wafer is first picked up by a robotic arm for transfer from one station to another station. Next, the wafer is placed on an aligner for aligning and centering the wafer to a desired position using a notch or flat located on the wafer's edge. Once properly aligned, the wafer is then placed in the desired station for processing. After the processing is completed at the desired station, the wafer may then be picked up and placed again at another station.
Each time that the wafer is picked up, placed, and aligned, contact is made with either the edge or the back side of the wafer and particles are generated. For instance, in a single wafer process cycle, the wafer may be contacted as many as twelve times when using a three-axis aligner or at least eight times when a single axis aligner is used.
In addition, the alignment process requires a dedicated aligning device and a separate step in the wafer process cycle. The dedicated aligning device often creates a bottleneck that limits the wafer throughput in the system and also introduces additional handling that generates particles. Adding aligners to the system may help to slightly increase this throughput problem but creates an undesirable increase in the cost, complexity and generation of particles to the wafer handling system. Accordingly, a system is desired for enhancing the wafer handling process by reducing the generation of particles and wafer damage so that the wafer yield is increased. Also, it is desired to increase the wafer throughput by performing the alignment process in parallel with moving the wafer.
In order to effect alignment/reorienting of the wafer (or any other flat panel substrate/workpiece) in parallel with moving the wafer so called “on the fly” alignment, the handling systems are generally provided with means or drives for performing such alignment. In conventional handling system, the “on the fly” alignment drives have generally been used by systems that do not employ “edge gripping” for holding the substrate. These non-edge gripping systems, however, are generally falling into disfavor with users (for edge-gripping systems) because the non-edge gripping systems contact the wafer surface with the associated potential for contamination. In the case of edge gripping systems, attempts have been made to provide an “on the fly” alignment drive. These conventional attempts take the form of a servomotor mounted away from the wafer and the chuck holding the wafer on the handling system. This is known as “off axis” mounting. The servomotor is connected to the chuck by a suitable transmission that imparts motion from the servomotor to the chuck. In conventional systems, locating the motor away from the substrate minimizes potential for particulate contaminants generated during motor operation coming in contact with the wafer surfaces. Nevertheless, by employing a transmission to connect the motor to the chuck, such as cables, conventional systems still have the potential for having particles contaminate the surface of the wafers being handled. Further, the transmission, by its very nature as a further drive element between motor and chuck, increases the possibility for inaccuracy in placement/aligning of the substrate. The transmission further increases the weight and complexity of the end effector with a detrimental impact to the control of the handling system. Further, due to the very tight height constraints, the chuck, of the end effector is inserted between stacked wafers in a pod/cassette with a pitch of about 10.0 mm and a wafer thickness of about 0.77 mm, the transmission design is complex. Correspondingly, the transmission may be unreliable and time consuming to install. The present invention as evident from the exemplary embodiments overcomes the problems of conventional handling systems as will be described in greater detail below.